Open-cell expanded ceramic with a high level of strength, and process for the production thereof

ABSTRACT

The disclosed invention relates to the field of ceramics and concerns an open-cell expanded ceramic which may be used in the form of a deep-bed filter, and a process for the production thereof. The primary object is to produce an open-cell expanded ceramic by a simple and economical process. This object is attained by an open-cell expanded ceramic in which the inner cavities, cracks and the porosity of the ceramic members are filled partially or completely by one or a plurality of metal and/or ceramic phases and/or glass phases. The open-cell expanded ceramic is also produced in that during or after sintering the cavities, cracks and the porosity of the ceramic members are partially or completely filled with a melt or a suspension which melt below the melting temperature of the expanded ceramic, have a coefficient of expansion similar to the coefficient of expansion of the expanded ceramic, and a very good wetting capacity, and only react partially or not at all with constituents of the expanded ceramic.

BACKGROUND OF THE INVENTION

a) Field of the Invention

The invention relates to the field of ceramics and is directed to anopen-cell expanded ceramic with high strength such as can be used, e.g.,for deep-bed filters, supporting bodies for filtration, heat exchangers,regenerators, electrically heatable thermostats, catalytic substrates orsupports, burner elements for surface radiant burners and volumeburners, high temperature reaction chambers, acoustic dampening orreinforcing material for panels for high temperature, and to a processfor the production thereof.

b) Description of the Related Art

Processes are known for the production of open-cell expanded ceramics bythe Schwartzwalder method, as it is called, which is utilizedcommercially and is most common. According to this process, the desiredstructural component part is cut out of an open-cell polymer foam andsubsequently impregnated with a suspension of ceramic particles andwater or solvent. Afterward, the impregnated polymer foam is repeatedlypressed out mechanically and then dried. The polymer foam is then burnedout and the sintering of the remaining ceramic coating is subsequentlycarried out (U.S. Pat. No. 3,090,094).

Open-cell expanded ceramic produced according to this process is acasting of the cell-like polymer structure of the starting material.Because the polymer foam is burned out, the remaining ceramic webs orstruts are hollow.

These struts have a three-edged cross section and the cavities also havea three-edged shape in cross section. The ceramic coating is frequentlycracked at the edges of the cavities. The cavities and cracks result invery low mechanical strength. Since the contraction of the ceramiccoating during sintering further increases susceptibility to cracking,relatively low-contraction substances are used, although the latter havea high internal porosity after sintering. This likewise leads to a lowmechanical strength (J. A. Ceram. Soc. 77 (6), 1467-72 (1994)).

In order to increase strength, it has already been attempted to providethe open-cell expanded ceramic with greater strength by applying one ormore subsequent coats to the ceramic struts of the foam before or aftersintering the ceramic foam. These subsequent coats are applied byimpregnating either the coated polymer foam or the sintered expandedceramic with a ceramic slurry (suspension) (e.g., GB 2097777).

A problem consists in that the excess suspension cannot be pressed outof the cells of the expanded ceramic mechanically without destroying thenow rigid foam structure; for this reason, very thin suspensions with alow solid content must be used which can drip without leading to aclosure of the cells and accordingly to a reduction in the number ofopen cells. Thin suspensions with low solid content have thedisadvantage that the coating of ceramic particles is only thin and thiscoating is interrupted during the drying of the suspension by dryingcracks or, during the sintering, by contraction cracks.

Further, the efficacy of an external multiple coating of the cell strutsis only slight because the unfavorable structure of the cavities in thestruts is not overcome. Further, a subsequent coating reduces the volumeof the free cells of the expanded ceramic, which is disadvantageous formost applications (J. Am. Ceram. Soc. 77 (6), 1467-72 (1994), page 1467,second paragraph, left).

A process is known from EP 0369 098 wherein a presintered open-cellceramic foam is impregnated with a suspension of colloidal refractoryoxide and a refractory oxide powder under a vacuum and, after thesuspension drips off, is dried and sintered. Accordingly, in addition toa coating of the cell struts (as was mentioned above), the cavities ofthe cell struts must also be filled with the suspension duringimpregnation. As was already described above, a very thin suspensionmust be used for the impregnation, so that only a very small proportionof ceramic particles can reach the cavities of the struts during theimpregnation. Therefore, the proportion by volume of the ceramic phaseafter sintering is only extremely small. Further, as was mentionedabove, the layers of low-concentration ceramic suspension crackrepeatedly during drying and sintering, so that their reinforcingefficiency is further diminished.

It is also known to produce an open-cell carbon foam by means ofCVD/CVI. In this case, the carbon foam is produced by pyrolysis of anopen-cell polymer foam and is subsequently cut according to the desiredgeometry of the structural component part. The carbon foam is thencoated with ceramic components by means of CVD/CVI (Ceram. Bull. Vol.70, No. 6,1991, 1025-1028). A production process of this kind is veryexpensive and requires elaborate plant technology. The expanded ceramicproduced by this process is also a casting of the cell-like polymerstructure of the starting material. The ceramic struts are formed of atight layer of the applied ceramic, wherein the struts are formedinternally by the original carbon skeleton. Due to the tightness andstrength of the ceramic coating, the expanded ceramic is very strong onthe whole but, because of the internal carbon skeleton which hassubstantially lower strength than the ceramic layer, the mechanicalstrength is still not adequate under high loading. Further, the carbonskeleton is exposed to oxidative processes at high temperatures whichgreatly undermines the otherwise favorable stability of the expandedceramic at high temperatures. For these reasons, the carbon skeleton isremoved for applications at higher temperatures, but this again resultsin hollow ceramic struts.

Another known method for producing an open-cell expanded ceramic isdirect expansion using foaming agents. For this purpose, a suspension ofceramic particles and water or a solvent is first produced. A foamingagent and polymer components are added to this suspension. Thissuspension is subsequently cast in a mold and the reaction of thefoaming agent is initiated. This reaction brings about the developmentof gas bubbles which cause a foaming of the suspension. The polymercomponents are then cross-linked, so that the foam hardens. The polymercomponents are then burned out and the remaining foam is sintered(Product Brochure: Foaming Agents W 53 FL, Zschimmer & Schwartz GmbH &Co., Lahnstein). The disadvantage of this process consists in that thefoaming is difficult to control.

An expanded ceramic produced by this process has a net-like structure.The ceramic struts are composed of ceramic through the entire crosssection after sintering. The external geometry of an expanded ceramic ofthis type is limited at least in one dimension by the open mold in whichthe foaming takes place. The sintered expanded ceramic is verymicroporous because the suspension does not allow a high concentrationof particles. At higher concentrations, the suspension is too heavy andfoaming takes place only incompletely or not at all. Low solidconcentrations lead to a very loose structure of the burned out ceramic.When the ceramic is compressed by contraction during sintering, stressesand cracks also occur, so that the strength of the ceramic is limited.If a ceramic system with low contraction is selected, the porosity ismaintained which likewise results in low strength. Further, it isdisadvantageous that the pore spacing is difficult to control in generaland especially over the height of the mold which impairs thethrough-flow capacity of the expanded ceramic.

Further, a process for the production of expanded ceramic through directexpansion by means of air is also known. For this purpose, a polymercomponent is added to a suspension of ceramic particles and water or asolvent. Subsequently, air bubbles are introduced into the suspension bya high-speed special stirrer. The foamy suspension is then poured into amold and the foam is hardened by cross-linking the polymer component.The polymer component is then burned out and the foam is sintered.

Only very fine foams with few open cells can be produced by the processmentioned above. The struts are formed of ceramic material over theentire cross section. The strength of these foams is likewise limitedbecause a high concentration of particles is not possible and stressesand cracks which limit the strength of the expanded ceramic likewiseoccur during contraction while sintering. Further, the flow through thefoam is also impaired in this case because the “pores” are often closedcells.

OBJECT AND SUMMARY OF THE INVENTION

It is the primary object of the invention to produce an open-cellexpanded ceramic with improved strength by a simple and economicalprocess.

This object is met in accordance with the invention by an open-cellexpanded ceramic with high strength comprising ceramic struts havinginner cavities, cracks and pores and wherein the inner cavities, cracksand pores are filled at least partially with at least one of a groupconsisting of at least one metal phase, at least one ceramic phase andat least one glass phase.

In the open-cell expanded ceramic with high strength according to theinvention, the inner cavities of the ceramic struts, the cracks of theceramic struts and the pores of the ceramic struts are filled completelyor partially with one or more metallic phases and/or ceramic phasesand/or glass phases.

The inner cavities of the ceramic struts, the cracks of the ceramicstruts and the pores of the ceramic struts are advantageously filledcompletely or partially with one or more metallic phases.

The inner cavities of the ceramic struts, the cracks of the ceramicstruts and the pores of the ceramic struts are likewise advantageouslyfilled completely or substantially completely with one or more ceramicphases.

It is also advantageous when the inner cavities of the ceramic struts,the cracks of the ceramic struts and the pores of the ceramic struts arefilled completely or substantially completely with one or more glassphases.

It is also advantageous when the inner cavities of the ceramic struts,the cracks of the ceramic struts and the pores of the ceramic struts arefilled completely or substantially completely with silicon or siliconcompounds, with molybdenum or metal silicides or with aluminum yttriumoxide or with calcium silicates or strontium calcium silicates orfluorides for silicon carbide expanded ceramics, with copper-titaniumalloys or iron-titanium alloys or with titanium carbide for aluminumoxide expanded ceramics, with mullite for zirconium oxide expandedceramics.

In a further advantageous variant of the invention, the inner cavitiesof the ceramic struts, the cracks of the ceramic struts and the pores ofthe ceramic struts of silicon carbide expanded ceramics are filledcompletely or substantially completely with silicon and/or siliconcompounds.

Further, the open-cell expanded ceramics with high strength according tothe invention, whose inner cavities of the ceramic struts, cracks of theceramic struts and pores of the ceramic struts are filled completely orpartially with one or more metallic phases and/or ceramic phases and/orglass phases is produced, according to the invention, in that anopen-cell polymer foam is cut to size, coated with a suspension ofceramic particles and water or a solvent, pressed out and dried, thepolymer foam is then burned out or pyrolized, the coated foam issubsequently sintered and, during or after the sintering, the cavitiesof the ceramic struts, the cracks of the ceramic struts and the pores ofthe ceramic struts of the sintered expanded ceramics are filledcompletely or partially with a melt or a suspension, wherein the meltand the suspension comprise materials which melt below the meltingtemperature of the expanded ceramics, have a coefficient of expansionsimilar to that of the expanded ceramics, exhibit very good wetting andreact partially or not at all with components of the expanded ceramic,and wherein, when the struts are filled with a suspension, the expandedceramic filled with the suspension is subsequently heated to atemperature above the melting temperature of the materials, mixtures ofmaterials or reaction products thereof contained in the suspension.

The cavities of the ceramic struts, the cracks of the ceramic struts andthe pores of the ceramic struts of the sintered expanded ceramic areadvantageously filled with a melt comprising silicon or siliconcompounds, molybdenum or metal silicides or aluminum yttrium oxide orcalcium silicates or strontium calcium silicates or fluorides forsilicon carbide expanded ceramics, copper-titanium alloys oriron-titanium alloys or titanium carbide for aluminum oxide expandedceramics, mullite for zirconium oxide expanded ceramics.

Also, the cavities of the ceramic struts, the cracks of the ceramicstruts and the pores of the ceramic struts of the sintered expandedceramic are advantageously filled with a melt comprising silicon orsilicon compounds, aluminum, boron, iron, copper or oxygen for siliconcarbide expanded ceramics.

Another advantageous variant of the invention consists in that thecavities of the ceramic struts, the cracks of the ceramic struts and thepores of the ceramic struts of the sintered expanded ceramic are filledwith a suspension which, in addition to water or a solvent, comprisespowder of silicon or silicon compounds, molybdenum or metal suicides oraluminum yttrium oxide or calcium silicates or strontium calciumsilicates or fluorides for silicon carbide expanded ceramics,copper-titanium alloys or iron-titanium alloys or titanium carbide foraluminum oxide expanded ceramics, mullite for zirconium oxide expandedceramics. After the cavities of the ceramic struts, the cracks of theceramic struts and the pores of the ceramic struts are filled with thesuspension, drying is carried out. Accordingly, the powder contained inthe suspension and/or its reaction products are located in the cavities,cracks and pores. Immediately after drying or later, this sinteredexpanded ceramic with the powder and/or its reaction products located inthe cavities, cracks or pores is heated to a temperature above themelting temperature of the powder components or their reaction products.Accordingly, the powder components and/or their reaction products aremelted. This melt then completely or partially fills the cavities,cracks and pores of the sintered expanded ceramic and hardens thereinwhen cooled.

It is also advantageous when the cavities of the ceramic struts, thecracks of the ceramic struts and the pores of the ceramic struts of thesintered expanded ceramic are filled with a suspension which, inaddition to water or a solvent, contains powder of silicon or siliconcompounds with aluminum, boron, iron, copper or oxygen for siliconcarbide expanded ceramics. The suspension is then likewise dried and,immediately after drying or later, is heated to a temperature at whichthe powder components and/or reaction products melt.

It is also advantageous when the cavities of the ceramic struts, thecracks of the ceramic struts and the pores of the ceramic struts of thesintered expanded ceramic are filled with a suspension which, inaddition to water or a solvent, contains powder of glass frit. Thesuspension is then likewise dried and immediately after drying or lateris heated to a temperature at which the powder components and/orreaction products melt.

It is also advantageous to use glass frit comprising frit of one or moreboron silicate glasses, aluminum boron silicate glasses and/or lithiumaluminum silicate glasses.

It is particularly advantageous when the utilized materials of which themelt is comprised or which are contained in the suspension have acontact angle of 0 to 500 in the molten state.

In another possible arrangement of the invention, the utilized materialsof which the melt is comprised or which are contained in the suspensionreact partially with components of the expanded ceramic in the moltenstate and accordingly lead to a reaction bonding with the expandedceramic.

The process according to the invention can advisably also be arranged insuch a way that the filling of the inner cavities of the ceramic struts,cracks of the ceramic struts and pores of the ceramic struts is carriedout by means of melt infiltration.

The melt infiltration is advantageously carried out by wickinfiltration, bulk infiltration or paste infiltration.

The expanded ceramic finally comprises the ceramic coating which remainsafter the polymer foam is burned out. According to the invention, a meltor a suspension is introduced into the inner cavities of the ceramicstruts and the cracks of the ceramic struts of this expanded ceramic.

As a result of the solution according to the invention, an open-cellexpanded ceramic with high strength and uniform pore structure can beproduced by a simple and economical process. This is achieved in thatthe former cavities, cracks and pores in the ceramic struts are filledwith a melt which then solidifies.

Further, the ceramic struts can also be filled with a suspension. Theexpanded ceramic which is filled in this way is then heated to atemperature which lies above the melting temperature of the materials,mixtures of materials or reaction products thereof contained in thesuspension. The powder components which have been contained in thesuspension are melted and, due to the high wetting of the ceramic withthe melt, a redistribution is brought about in such a way that the innercavities of the ceramic struts, the cracks of the ceramic struts and thepores of the ceramic struts are completely or substantially completelyfilled with the melt. Cracks which may possible occur in the driedsuspension are accordingly eliminated.

When this variant of the invention is realized, after the inner cavitiesof the ceramic struts, the cracks of the ceramic struts and the pores ofthe ceramic struts are filled, only a partial filling of these areaswith powder and/or its reaction products is initially carried out inaccordance with the solid content of the suspension that is used.However, in this process variant, the ceramic struts are coated tovarying degrees of thickness at the outer surface with a suspension atthe same time as the inner cavities of the ceramic struts, the cracks ofthe ceramic struts and the pores of the ceramic struts are filled. Afterdrying, a variously thick layer of powder components and/or theirreaction products is present at the outer surfaces of the ceramicstruts. When temperature is increased to a temperature above the meltingtemperature of the powder components and/or their reaction products,these layers also melt and completely or substantially completely fillthe inner cavities of the ceramic struts, the cracks of the ceramicstruts and the pores of the ceramic struts through the accesses to thecavities and the cracks and through the pores in the ceramic struts.

Viscous-to-pasty suspensions which cause the ceramic struts to bethickly coated, as well, can also be used to fill the inner cavities ofthe ceramic struts, the cracks of the ceramic struts and the pores ofthe ceramic struts. Even a partial closure of the cells of the expandedceramic by the suspension is not troublesome, as this is eliminatedagain after the melting of the powder components contained in thesuspension and the distribution of the melt in the inner cavities of theceramic struts, the cracks of the ceramic struts and the pores of theceramic struts.

An expended ceramic produced in this way has a very uniform structurewith respect to cell size and strut thickness, has open cells, is verystrong and can be used near the melting temperature of the low-meltingphase in the expanded ceramic.

When the filling material contains components reacting with the ceramicof the foam, this results, first, in a strengthening of the ceramiccovering by filling the pores with reaction products and additionalbonding of the ceramic particles. Second, the reaction reinforces thefilling of the interior of the struts through the formation of reactionfronts and superelevated temperatures.

DESCRIPTION OF THE PREFERRED EMBODIMENTS AND BEST MODE FOR CARRYING OUTTHE INVENTION

The invention will be described hereinafter with reference to severalembodiment examples.

EXAMPLE 1

A ceramic water-based slurry is produced with a bimodal SiCdistribution, with grain size maxima of 1 and 20 μm and a solid contentof 55-65 percent by volume. A piece of polyurethane foam (10 ppi) withdimensions of 40×40×25 mm is impregnated with this slurry. The excessslurry is separated by means of a centrifuge to a mass of 15 g. Thecoated foam is then dried and the polyurethane is burned out at 600° C.The remaining SiC expanded ceramic is sintered at 2300° C. under anargon gas atmosphere. After cooling, wick infiltration with liquidsilicon into the inner cavities, cracks and pores of the ceramic strutsis carried out under a vacuum. After the silicon hardens, there resultsa SiC expanded ceramic in which the inner cavities of the ceramicstruts, the cracks of the ceramic struts and the pores of the ceramicstruts are filled with silicon. The average load at fracture whenindented by an indenter having a diameter of 20 mm is 1500 N at a massof 19 g. The load at fracture of an expanded foam of comparable SiCmaterial (ceramically bonded or liquid-phase sintered) without fillingof the pores and cavities of the ceramic struts is approximately 300 Nwith the same coating thickness.

EXAMPLE 2

A ceramic water-based slurry is produced with a bimodal SiCdistribution, with grain size maxima of 1 and 20 μm and a solid contentof 55-65 percent by volume. A piece of polyurethane foam (10 ppi) withdimensions of 40×40×25 mm is impregnated with this slurry. The excessslurry is separated by means of a centrifuge to a mass of 15 g. Thecoated foam is then dried and the polyurethane is burned out at 600° C.The remaining SiC expanded ceramic is sintered at 2300° C. under anargon gas atmosphere. Further, a ceramic waterbased slurry is producedwith 50 percent by volume of Si powder (d₅₀=10 μm). The sinteredexpanded ceramic is impregnated with this slurry and the excesssuspension is centrifuged off in a centrifuge to a weight ofapproximately 19 g. The impregnated sintered expanded ceramic is thendried and heated to 1600° C. under a vacuum. The applied silicon meltsand saturates the SiC expanded ceramic. After the silicon hardens, thereresults a SiC expanded ceramic in which the inner cavities of theceramic struts, the cracks of the ceramic struts and the pores of theceramic struts are filled with silicon. The average load at fracturewhen indented by an indenter having a diameter of 20 mm is 1500 N at amass of 19 g.

EXAMPLE 3

A ceramic water-based slurry is produced with a bimodal SiCdistribution, with grain size maxima of 1 and 20 μm and a solid contentof 55-65 percent by volume. A piece of polyurethane foam (40 ppi) withdimensions of 50×50×10 mm is impregnated with this slurry. The excessslurry is removed by squeezing by means of two rubber rollers. Thecoated foam is then dried. The polyurethane is pyrolized and 20 to 40percent by mass, in relation to the uninfiltrated coated foam, of liquidsilicon is simultaneously infiltrated into the inner cavities, cracksand pores of the ceramic struts under a vacuum at 1600° C. in oneprocess step. After the silicon hardens, there results a SiC expandedceramic in which the inner cavities of the ceramic struts, the cracks ofthe ceramic struts and the pores of the ceramic struts are filled withsilicon. The average load at fracture when indented by an indenterhaving a diameter of 20 mm is 1000 N at a mass of 18 g

EXAMPLE 4

A ceramic water-based slurry is produced with a SiC powder with grainsize maxima of 3 μm and carbon black (4% C in relation to SiC) with asolid content of 55-65 percent by volume. A piece of polyurethane foam(10 ppi) with dimensions of 40×40×25 mm is impregnated with this slurry.The excess slurry is separated by means of a centrifuge to a mass of 15g and the coated expanded ceramic is dried. The polyurethane ispyrolized and 20 to 40 percent by mass, in relation to the uninfiltratedcoated foam, of liquid silicon is simultaneously infiltrated under avacuum at 1600° C. in one process step. The silicon initially reactswith the carbon black in the strut coatings to form secondary SiC whichadditionally bonds the existing SiC grains. The rest of the pores, innercavities and cracks are subsequently filled with silicon. After thesilicon hardens, there results a SiC expanded ceramic in which the innercavities of the ceramic struts, the cracks of the ceramic struts and thepores of the ceramic struts are filled with silicon. The average load atfracture when indented by an indenter having a diameter of 20 mm is 1300N at a mass of 19 g.

EXAMPLE 5

A ceramic water-based slurry is produced with a broad Al₂O₃ graindistribution (grain size maximum of 80 μm) and 10 percent by mass ofclay at a solid content of 80-70 percent by volume. A piece ofpolyurethane foam (10 ppi) with dimensions of 40×40×25 mm is impregnatedwith this slurry. The excess slurry is separated by means of acentrifuge to a mass of 15 g. The coated foam is then dried and thepolyurethane is burned out at 600° C. under air atmosphere. Theremaining Al₂O₃ expanded ceramic is sintered at 1350° C. Subsequently,an infiltration of the foam is carried out with a CuTi (20% Ti) alloy at1350° C. under a vacuum, wherein the CuTi alloy is added in liquid form.There results an expanded ceramic in which the inner cavities of theceramic struts, the cracks of the ceramic struts and the pores of theceramic struts are filled with a CuTi alloy. The outer covering of thestruts comprises Al₂O₃ particles and the CuTi alloy is located betweenthe particles. The average load at fracture when indented by an indenterhaving a diameter of 20 mm is 1200 N at a mass of 40 g.

EXAMPLE 6

A ceramic water-based slurry is produced with a solid content of60percent by volume. The solid in the slurry is composed of 80 percentSiC powder with a bimodal grain size distribution with two grain sizemaxima of 6 and 20 μm and of 20 percent clay. A piece of polyurethanefoam (10 ppi) with a diameter of 70 mm and a height of 25 mm isimpregnated with this slurry. The excess slurry is separated by means ofa centrifuge to a mass of 35 g. The coated foam is then dried, thepolyurethane is burned out and the ceramic is sintered in a chamberfurnace at 1200° C. under air atmosphere. Immediately thereafter,saturation of the sintered expanded ceramic is carried out with anaqueous suspension with 60 percent by volume of a mixture of 85% finelydispersed SiO2 and 15% sodium silicate. The excess suspension iscarefully centrifuged off in the above-mentioned centrifuge to a massincrease of 25% and the saturated foam is dried. A through-type secondburning is then carried out at 1200° C. After burning, there results aclay-bonded silicon carbide expanded ceramic in which the inner cavitiesof the ceramic struts and the cracks of the ceramic struts are almostcompletely filled with a silicate-type glass. The average load atfracture when indented by an indenter having a diameter of 20 mm is 800N. In contrast, a specimen without filled struts only achieved a load atfracture of 500 N.

EXAMPLE 7

A ceramic water-based slurry is produced with a solid content of 78percent by mass. The solid of the slurry comprises a commercial Al₂O₃sinter mixture (96-%, average grain size 5 μm). A piece of polyurethanefoam (30 ppi) with dimensions of 125×40×20 mm is impregnated with thisslurry. The excess slurry is separated by means of a centrifuge to amass of 60 g. The coated foam is then dried, the polyurethane is burnedout and the ceramic is burned in the chamber furnace at 1650° C. underair atmosphere. Immediately thereafter, saturation of the sinteredexpanded ceramic is carried out with an aqueous suspension with 60percent by volume of a finely ground frit of aluminum boron silicateglass. The excess suspension is centrifuged off in the above-mentionedcentrifuge until reaching a mass increase of 30% and the saturated foamis dried. A second burning is then carried out at 1200° C. Afterburning, there results an aluminum oxide ceramic in which the innercavities of the ceramic struts and the cracks of the ceramic struts arealmost completely filled with a silicate-type glass. The average load atfracture when indented by an indenter having a diameter of 20 mm is 2000N. In contrast, specimens without filled struts only achieved an averageload at fracture of 1500 N.

While the foregoing description and drawings represent the presentinvention, it will be obvious to those skilled in the art that variouschanges may be made therein without departing from the true spirit andscope of the present invention.

What is claimed is:
 1. An open-cell expanded ceramic comprising ceramicstruts having inner cavities, cracks and pores and wherein said innercavities, cracks and pores of the ceramic struts are filled at leastpartially with at least one of a group consisting of at least onemetallic phase, at least one ceramic phase and at least one silicateglass phase; wherein the inner cavities of the ceramic struts, thecracks of the ceramic struts and the pores of the ceramic struts ofsilicon carbide expanded ceramics are filled completely with at leastone of silicon and silicon compounds.
 2. The open-cell expanded ceramicaccording to claim 1, wherein the inner cavities of the ceramic struts,the cracks of the ceramic struts and the pores of the ceramic struts arecompletely filled with at least one metallic phase.
 3. The open-cellexpanded ceramic according to claim 1, wherein the inner cavities of theceramic struts, the cracks of the ceramic struts and the pores of theceramic struts are completely filled with at least one ceramic phase. 4.The open-cell expanded ceramic according to claim 1, wherein the innercavities of the ceramic struts, the cracks of the ceramic struts and thepores of the ceramic struts are at least partially filled with at leastone glass phase.
 5. The open-cell expanded ceramic according to claim 1,wherein the inner cavities of the ceramic struts, the cracks of theceramic struts and the pores of the ceramic struts are filled withsilicon or silicon compounds, with molybdenum or metal silicides or withaluminum yttrium oxide or with calcium silicates or strontium calciumsilicates or fluorides for silicon carbide expanded ceramics, withcopper-titanium alloys or iron-titanium alloys or with titanium carbidefor aluminum oxide expanded ceramics, with mullite for zirconium oxideexpanded ceramics.
 6. A process for the production of an open-cellexpanded ceramic comprising the steps of: cutting an open-cell polymerfoam to size; coating the polymer foam with a suspension of ceramicparticles in water or in a solvent; pressing the polymer foam out anddrying it; burning out or pyrolizing the polymer foam; subsequentlysintering the ceramic particles to form a sintered expanded ceramic withstruts, cavities within the struts, cracks of the ceramic struts, andpores of the ceramic struts; during or after the sintering, filling thecavities within the ceramic struts, the cracks of the ceramic struts andthe pores of the ceramic struts of the sintered expanded ceramics atleast partially with a melt or a suspension; wherein the melt and thesuspension comprise materials which melt below a melting temperature ofthe expanded ceramic, have a coefficient of expansion similar to that ofthe expanded ceramic exhibit very good wetting and react partially ornot at all with components of the expanded ceramic; and when the strutsare filled with a the suspension, subsequently heating the expandedceramic filled with the suspension to a temperature above the meltingtemperature of materials, mixtures of materials or reaction productsthereof contained in the suspension; wherein the cavities of the ceramicstruts, the cracks of the ceramic struts and the pores of the ceramicstruts of the sintered expanded ceramic are filled with a meltcomprising silicon or silicon compounds, molybdenum or metal silicidesor aluminum yttrium oxide or calcium silicates or strontium calciumsilicates or fluorides for silicon carbide expanded ceramics,copper-titanium alloys or iron-titanium alloys or titanium carbide foraluminum oxide expanded ceramics, mullite for zirconium oxide expandedceramics.
 7. The process according to claim 6, wherein the cavities ofthe ceramic struts, the cracks of the ceramic struts and the pores ofthe ceramic struts of the sintered expanded ceramic are filled with amelt comprising silicon or silicon compounds, aluminum, boron, iron,copper or oxygen for silicon carbide expanded ceramics.
 8. The processaccording to claim 6, wherein the cavities of the ceramic struts, thecracks of the ceramic struts and the pores of the ceramic struts of thesintered expanded ceramic are filled with a suspension which, inaddition to the water or a the solvent, contains powder of silicon orsilicon compounds with aluminum, boron, iron, copper or oxygen forsilicon carbide expanded ceramics.
 9. The process according to claim 6,wherein the utilized materials of which the melt is comprised or whichare contained in the suspension have a contact angle of 0 to 50° in themolten state.
 10. The process according to claim 6, wherein the utilizedmaterials of which the melt is comprised or which are contained in thesuspension react partially with components of the expanded ceramic inthe molten state and accordingly lead to a reaction bonding with theexpanded ceramic.
 11. The process according to claim 6, wherein thefilling of the inner cavities of the ceramic struts, cracks of theceramic struts and pores of the ceramic struts is carried out by meansof melt infiltration.
 12. The process according to claim 11, wherein themelt infiltration is carried out by at least one of wick infiltration,bulk infiltration and paste infiltration.
 13. A process for theproduction of an open-cell expanded ceramic comprising the steps of:cutting an open-cell polymer foam to size; coating the polymer foam witha suspension of ceramic particles in water or in a solvent; pressing thepolymer foam out and drying it; burning out or pyrolizing the polymerfoam; subsequently sintering the ceramic particles to form a sinteredexpanded ceramic with struts, cavities within the struts, cracks of theceramic struts, and pores of the ceramic struts; during or after thesintering, filling the cavities within the ceramic struts, the cracks ofthe ceramic struts and the pores of the ceramic struts of the sinteredexpanded ceramics at least partially with a melt or a suspension;wherein the melt and the suspension comprise materials which melt belowa melting temperature of the expanded ceramic, have a coefficient ofexpansion similar to that of the expanded ceramic exhibit very goodwetting and react partially or not at all with components of theexpanded ceramic; and when the struts are filled with a the suspension,subsequently heating the expanded ceramic filled with the suspension toa temperature above the melting temperature of materials, mixtures ofmaterials or reaction products thereof contained in the suspension;wherein the cavities of the ceramic struts, the cracks of the ceramicstruts and the pores of the ceramic struts of the sintered expandedceramic are filled with a suspension which, in addition to the water orthe solvent, comprises powder of silicon or silicon compounds,molybdenum or metal silicides or aluminum yttrium oxide or calciumsilicates or strontium calcium silicates or fluorides for siliconcarbide expanded ceramics, copper-titanium alloys or iron-titaniumalloys or titanium carbide for aluminum oxide expanded ceramics, mullitefor zirconium oxide expanded ceramics.
 14. A process for the productionof an open-cell expanded ceramic comprising the steps of: cutting anopen-cell polymer foam to size; coating the polymer foam with asuspension of ceramic particles in water or in a solvent; pressing thepolymer foam out and drying it; burning out or pyrolizing the polymerfoam; subsequently sintering the ceramic particles to form a sinteredexpanded ceramic with struts, cavities within the struts, cracks of theceramic struts, and pores of the ceramic struts; during or after thesintering, filling the cavities within the ceramic struts, the cracks ofthe ceramic struts and the pores of the ceramic struts of the sinteredexpanded ceramics at least partially with a melt or a suspension;wherein the melt and the suspension comprise materials which melt belowa melting temperature of the expanded ceramic, have a coefficient ofexpansion similar to that of the expanded ceramic exhibit very goodwetting and react partially or not at all with components of theexpanded ceramic; and when the struts are filled with a the suspension,subsequently heating the expanded ceramic filled with the suspension toa temperature above the melting temperature of materials, mixtures ofmaterials or reaction products thereof contained in the suspension;wherein the cavities of the ceramic struts, the cracks of the ceramicstruts and the pores of the ceramic struts of the sintered expandedceramic are filled with a suspension which, in addition to the water orthe solvent, contains powder of glass frit.
 15. The process according toclaim 14, wherein glass frit comprising frit of one or more boronsilicate glasses, aluminum boron silicate glasses and/or lithiumaluminum silicate glasses is used.